1. Field of the Invention
This invention relates to an electric lamp having an infrared reflecting filter and a light-scattering coating. More particularly, this invention relates to a lamp comprising a light-transmissive envelope enclosing an electric source of light with an infrared reflecting filter on the envelope and a light-scattering coating disposed adjacent the filter, wherein the coating contains light-scattering particles dispersed in a silica-containing, glassy binder.
2. Background of the Disclosure
Electric lamps employing a filament or arc as the source of light emit light in a pattern in which the light source is visible, unless the emitted light is broken up. Means commonly employed to break up a light source image include a lenticuled lens (with or without the use of a reflector), sandblasting the lamp envelope, etching the lamp envelope or coating the lamp envelope to scatter the emitted light and diffuse the light source image of the arc or filament. Acid etching or coating the lamp envelope is most commonly found in a conventional household type of incandescent lamp wherein the glass envelope enclosing the filament is acid etched (frosted) and/or coated with a particulate, light-diffusing material. A mixture of clay and silica or titanium dioxide is often used as the particulate, light-scattering material, because of its availability, light-scattering properties, chemical inertness and ability to withstand the high temperatures reached during lamp operation.
High intensity incandescent lamps used for stage and studio lighting employ a fused quartz filament or arc chamber and are sometimes sand-blasted to break up the emitted light source image so that the light emitted by the lamp is more diffuse and the light source is not visible on the object(s) being illuminated. The fused quartz is able to withstand the extremely high temperatures often exceeding 900.degree. C. reached during lamp operation.
More recently high intensity lamps have been developed having multilayer optical interference filters disposed on the outer surface of the vitreous lamp envelope containing the light source within. These filters are multilayer films or coatings which are designed to transmit visible light radiation, but reflect emitted infrared radiation (IR) back to the filament light source. The reflected IR radiation increases lamp efficacy by reducing the amount of electrical energy needed to maintain the desired filament temperature. Since the amount of visible light radiation is the same but the amount of electricity required to produce this radiation is reduced, the efficacy or visible light output per unit of electrical energy or lumens per watt (LPW), is increased. However, an infrared reflecting, multilayer optical interference filter cannot be applied to a rough surface, such as a light-scattering sand-blasted surface, because it won't be able to reflect infrared radiation emitted by the light source back to the light source. In order for such a filter to be useful the light source must have its optical axis aligned coincident with the optical axis of a lamp envelope whose surface is smooth and concentric about the light source, so that the filter reflects the emitted infrared radiation back to the light source, such as a filament. A rough surface will destroy the ability of the filter to reflect the emitted infrared radiation back to the filament or arc, thereby rendering it useless. If the surface roughness of the lamp envelope is large with respect to the wavelength of IR radiation (i.e., 4 microns), the IR is coherently reflected in a diffuse manner and misses the filament. If the roughness is on a scale of about 0.4-2 microns, then the coherent reflection is disrupted and the filter no longer reflects the IR since it is no longer a quarter wave filter. Thus, a need exists for a lamp having a filter which transmits visible light radiation but reflects infrared light radiation back to the light source and wherein the emitted light is broken up or scattered to diffuse the light source image making it less discernible.